The present invention relates to an objective lens used for an optical pick-up device and an optical pick-up device, and in particular, to an objective lens for performing recording/reproduction for optical information recording media which are different in recording density, and an optical pick-up device.
At present, there exist many kinds of optical information recording media, and the standards of these optical information recording media are established as shown in Table 1. Incidentally, hereafter (including lens data in the table), the power value of ten (for example, 2.5xc3x9710xe2x88x923) is expressed by E (for example, 2.5xc3x97Exe2x88x923).
Here, as the optical information recording media which are different in the recording density and required to have an interchangeability each other, there are CD and DVD. As shown in TABLE 1, these optical information recording media are different in the thickness of the transparent base board. In order to secure the interchangeability, it is necessary to correct, by any means, a spherical aberration generated by the difference of the thickness of the transparent base boards. Further, since DVD and CD are different each other in the required numerical aperture, some measures are necessary.
As the example which secures the interchangeability between DVD and CD by the objective lens, the objective lens having therein a diffractive structure is developed. As the above-mentioned objective lens, for example, there is the one in which the different diffractive structures are provided at the inside and outside of the specified distance h from the optical axis on one of the surfaces of the optical lens, and the spherical aberration is corrected for the thickness of each of the transparent base boards in an inside area, while the spherical aberration is corrected only for DVD in an outside area, and the spherical aberration is not corrected for CD, but is flared. By constructing the objective lens in the manner mentioned above, it is possible to form appropriately a light converging spot that is required for recording or reproducing of information on each optical information recording medium.
Here, concerning the flare on the outside area when CD is used, the case of an over flare and the case of an under flare are conceivable. In the case of under flare, the design makes a diffractive efficiency to be strong, causing a tendency that diffractive pitch is narrow. Accordingly, it is preferable in the design that the over flare is adopted from the view point of the diffractive efficiency.
In the optical pick-up device, an optical detecting device is generally used for judging whether the recording or reproduction of information is performed certainly or not. The optical detecting device detects the information recording light that is reflected on the surface of the optical information recording medium, and based on the result of the detection, the optical detecting device judges whether an error is caused or not. Here, the normal optical detecting device is composed of a main light receiving section, and sub light receiving sections provided on both sides of the main light receiving section, and mainly the main light receiving section reads the information recording light, while the sub light receiving section detects a tracking error.
As mentioned above, in the case that an optical surface of the objective lens is divided into two areas and the spherical aberration is corrected for the thickness of each of the transparent base boards in the inside area, while the spherical aberration is corrected only for DVD in the outside area, and that the spherical aberration is not corrected but flared for CD, a boundary of the flare light does not cover the main light receiving section and a pair of the sub light receiving sections of the optical detecting device. Accordingly, it can be said that any special problem might not happen, as far as the optical information recording media and optical detecting device are manufactured on the basis of design values.
However, in the optical information recording media, from the problem of a parts-incorporating error, when there happens the so-called disk tilt meaning that an axial line of CD is tilted to the axial line of the light converging optical system, or the so-called lens shift meaning that both of the axial lines are shifted, there is the danger that there happens a phenomenon that the boundary of the flared light is changed or the form of the flared light is distorted. The optical detecting device has essentially a function by which the tracking error is watched on the basis of the difference of the amounts of light received by the paired sub light receiving sections. However, when the above-mentioned phenomenon happens, there is the danger that an error signal is outputted, even if the tracking error does not occur.
The present invention has been achieved in view of the above-mentioned problems, and the object of the present invention is to provide an objective lens for an optical pick-up device and an optical pick-up device which can perform the recording or reproduction of information adequately for the different optical information recording media, though the construction is simple.
The objective lens for the optical pick-up device mentioned in Structure (1) is one, having
a first light source having the wavelength of xcex1 and performing the recording or reproduction of information by radiating the light flux on a first optical information recording medium having the thickness of a transparent base board of t1,
a second light source having the wavelength of xcex2 (xcex1 less than xcex2) and performing the recording or reproduction of information by radiating the light flux on a second optical information recording medium having the thickness of the transparent base board of t2 (t1 less than t2),
a light converging optical system including an objective lens which converges the light flux irradiated from the above-mentioned first and second light sources on information recording surfaces through transparent base boards of the first and second optical information recording media, and
an optical detecting device,
wherein there are arranged three or more optical function areas in the direction to cross the optical axis,
diffractive structures are formed on an inner optical function area located at the side near the optical axis and an outer optical function area located at the side farther from the optical axis respectively,
the light flux passing through the inner optical function area is corrected in terms of spherical aberration caused, when conducting the recording or reproduction for the first and second optical information recording medium,
the light flux passing through the outer optical function area is corrected in terms of the spherical aberration caused, when conducting the recording or reproduction for the first optical information recording medium, and further, is flared, when conducting the recording or reproduction for the second optical information recording medium,
the spherical aberration of the light flux having passed through an intermediate optical function area located between the inner optical function area and the outer optical function area is discontinuous to the spherical aberration of the light flux having passed through the outer optical function area, when conducting the recording or reproduction for the second optical information recording medium,
further, the absolute value of the amount of-the spherical aberration of the light flux having passed through the intermediate optical function area and arriving at the second optical information recording medium, when conducting the recording or reproduction for the second optical information recording medium,
is greater than
the absolute value of the amount of the spherical aberration on the light flux having passed through the inner optical function area and arriving at the second optical information recording medium, when conducting the recording or reproduction for the second optical information recording medium, and the following formula is satisfied, when an image-forming magnification of the objective lens, in conducting the recording or reproduction of information for the first optical information recording medium, is set to m1.
xe2x88x920.1 less than m1 less than 0.1xe2x80x83xe2x80x83(1)
Incidentally, in the present specification, xe2x80x9cthe spherical aberration is corrected by the optical function areaxe2x80x9d means that the spherical aberration of the light flux (light source wavelength xcex) passing through the optical function area is not greater than 0.07 xcexrms. Further, xe2x80x9cthe spherical aberration is discontinuousxe2x80x9d includes every discontinuous condition which will be stated later referring to FIG. 1.
In the objective lens for the optical pick-up device mentioned in Structure (2), when an image-forming magnification of the objective lens, in conducting recording or reproduction of information for the second optical information recording medium, is set to m2, the following formula is satisfied.
|m1xe2x88x92m2| less than 0.05xe2x80x83xe2x80x83(2)
In the objective lens for the optical pick-up device mentioned in Structure (3), when wave front aberration of a first effective light flux radiated from the first light source, in conducting the recording or reproduction of information for the first optical information recording medium, is set to SA1, the following formula is satisfied.
|SA1| less than 0.01 xcex1 rmsxe2x80x83xe2x80x83(3)
In the objective lens for the optical pick-up device mentioned in Structure (4), when wave front aberration of a second effective light flux radiated from the second light source, in conducting the recording or reproduction of information for the second optical information recording medium, is set to SA2, the following formula is satisfied.
xe2x80x83|SA2| less than 0.01 xcex2 rmsxe2x80x83xe2x80x83(4)
In the objective lens for the optical pick-up device mentioned in Structure (5), when the amount of vertical spherical aberration of the light ray passing through the outer most section of the second effective light flux, in conducting the recording or reproduction of information for the second optical information recording medium, is LSA2h2, and when the amount of vertical spherical aberration of the light ray passing through the outer most section of the second effective light flux, in conducting the recording or reproduction of information for the first optical information recording medium, is LSA1h2,
|LSA2h2|+|LSA1h2| less than 0.01 mmxe2x80x83xe2x80x83(5)
In the objective lens for the optical pick-up device mentioned in Structure (6), the light flux passing through the intermediate optical function area is flared, when conducting the recording or reproduction of information for the second optical information recording medium.
In the objective lens for the optical pick-up device mentioned in Structure (7), the light flux (intermediate light flux) passing through the intermediate optical function area has the under spherical aberration, when conducting the recording or reproduction of information for the second optical information recording medium.
In the objective lens for the optical pick-up device mentioned in Structure (8), the light flux passing through the intermediate optical function area is corrected in terms of the spherical aberration, when conducting the recording or reproduction of information for the first optical information recording medium.
The optical pick-up device mentioned in Structure (9) is one, having
a first light source having the wavelength of xcex1 and performing the recording or reproduction of information by radiating the light flux on a first optical information recording medium having the thickness of a transparent base board of t1,
a second light source having the wavelength of xcex2 (xcex1 less than xcex2) and performing the recording or reproduction of information by radiating the light flux on a second optical information recording medium having the thickness of the transparent base board of t2 (t1 less than t2),
a light converging optical system including an objective lens which converges the light flux irradiated from the above-mentioned first and second light sources on information recording surfaces through transparent base boards of the first and second optical information recording media, and
an optical detecting device,
wherein in the above-mentioned objective lens,
there are arranged three or more optical function areas in the direction to cross the optical axis,
diffractive structures are formed on an inner optical function area located at the side near the optical axis and an outer optical function area located at the side farther from the optical axis respectively,
the light flux passing through the inner optical function area is corrected in terms of spherical aberration caused when the recording or reproduction for the first and second optical information recording medium is performed,
the light flux passing through the outer optical function area is corrected in terms of the spherical aberration caused when conducting the recording or reproduction for the first optical information recording medium, and further, is flared when conducting the recording or reproduction for the second optical information recording medium,
the spherical aberration of the light flux having passed through an intermediate optical function area located between the first optical function area and the second optical function area is discontinuous to the spherical aberration of the light flux having passed through the outer optical function area, when conducting the recording or reproduction for the second optical information recording medium,
further, the absolute value of the amount of the spherical aberration of the light flux having passed through the intermediate optical function area and arriving at the second optical information recording medium when conducting the recording or reproduction for the second optical information recording medium
is greater than
the absolute value of the amount of the spherical aberration on the light flux having passed through the inner optical function area and arriving at the second optical information recording medium when the recording or reproduction for the second optical information recording medium is performed, and the following formula is satisfied, when an image-forming magnification of the objective lens in conducting the recording or reproduction of information for the first optical information recording medium is set to m1.
xe2x88x920.1 less than m1 less than 0.1xe2x80x83xe2x80x83(6)
Incidentally, in the present specification, xe2x80x9cthe spherical aberration is corrected by the optical function areaxe2x80x9d means that the spherical aberration of the light flux (light source wavelength xcex) passing through the optical function area is not greater than 0.07 xcexrms. Further, xe2x80x9cthe spherical aberration is discontinuousxe2x80x9d includes every discontinuous condition which will be stated later referring to FIG. 1.
In the optical pick-up device mentioned in Structure (10), when an image-forming magnification of the objective lens in conducting recording or reproduction of information for the second optical information recording medium is set to m2, the following formula is satisfied.
|m1xe2x88x92m2| less than 0.05xe2x80x83xe2x80x83(7)
In the optical pick-up device mentioned in Structure (11), when wave front aberration of a first effective light flux radiated from the first light source when conducting the recording or reproduction of information for the first optical information recording medium is set to SA1, the following formula is satisfied.
|SA1| less than 0.01 xcex1 rmsxe2x80x83xe2x80x83(8)
In the optical pick-up device mentioned in Structure (12), when wave front aberration of a second effective light flux radiated from the second light source when conducting the recording or reproduction of information for the second optical information recording medium is set to SA2, the following formula is satisfied.
|SA2| less than 0.01 xcex2 rmsxe2x80x83xe2x80x83(9)
In the optical pick-up device mentioned in Structure (13), when the amount of vertical spherical aberration of the light ray passing through the outer most section of the second effective light flux in conducting the recording or reproduction of information for the second optical information recording medium is LSA2h2, and when the amount of vertical spherical aberration of the light ray passing through the outer most section of the second effective light flux in conducting the recording or reproduction of information for the first optical information recording medium is LSA1h2,
|LSA2h2|+|LSA1h2 less than 0.01 mmxe2x80x83xe2x80x83(10)
In the optical pick-up device mentioned in Structure (14), the light flux passing through the intermediate optical function area is flared, when conducting the recording or reproduction of information for the second optical information recording medium.
In the optical pick-up device mentioned in Structure (15), the light flux (intermediate light flux) passing through the intermediate optical function area has the under spherical aberration, when conducting the recording or reproduction of information for the second optical information recording medium.
In optical pick-up device mentioned in Structure (16), the light flux passing through the intermediate optical function area is corrected in terms of the spherical aberration, when conducting the recording or reproduction of information for the first optical information recording medium.
In the present specification, when xe2x80x9can optical function areaxe2x80x9d shown by spherical aberration is applicable to either one of the following conditions, it is regarded that the different optical function areas exist across the boundary of h.
(a) Spherical aberration is discontinuous at h (FIG. 1(a)).
(b) Spherical aberration is continuous at h, but the primary differential is discontinuous at h (FIG. 1(b)).
(c) Spherical aberration is discontinuous on a certain wavelength (FIG. 1(c)).
Further, an area, where each light flux divided by the above-mentioned conditions passes through, is regarded as xe2x80x9can optical function areaxe2x80x9d. Due to this, when a refraction section and a diffraction section (diffractive structure) exist on one surface of a lens, the different xe2x80x9coptical function areasxe2x80x9d exist across the boundary of the refraction section and the diffraction section (See FIGS. 2(a) and 2(c)). Still further, though the diffractive structure is formed on the whole surface, when the diffractive structures designed on the basis of the different object are co-existed, they are regarded as the different xe2x80x9coptical function areasxe2x80x9d by the above-mentioned condition (c) (See FIG. 2(b)). Still further, when the discontinuous section is formed on the one surface, they are regarded as the different optical surfaces, though an aspheric surface expressed by the same aspheric coefficient is formed on the other surface. Incidentally, it is necessary that three or more optical function areas are arranged (FIGS. 2(a)-2(c)).
Concerning xe2x80x9can under spherical aberration or over spherical aberrationxe2x80x9d, when the spherical aberration crosses the optical axis before the paraxial image point, it is called xe2x80x9cunderxe2x80x9d, while when the spherical aberration crosses the optical axis beyond the paraxial image point, it is called xe2x80x9coverxe2x80x9d, on the spherical aberration having the origin on a paraxial image point position as shown in FIG. 3.
xe2x80x9cA diffractive structurexe2x80x9d in the present specification means the section on the surface of the objective lens where there is provided a relief that has a function to converge or diverge the light flux by diffraction. As a form of the relief, there is known a form that is formed on the surface of objective lens OL as a ring-shaped zone that is nearly concentric on the center of an optical axis as shown in FIG. 2(b) for example, and when its section on a plane including the optical axis is viewed, each ring-shaped zone is formed to be like a saw-tooth. The relief includes the above-mentioned form, and the form is called xe2x80x9ca diffractive ring-shaped zonexe2x80x9d particularly.
In the present specification, the objective lens in the narrow sense means a lens that is located closest to the optical information recording medium side to face it, and has a light converging function, under the condition that the optical information recording medium is loaded to the optical pick-up device. The objective lens in the broad sense means a lens group that can be moved by an actuator together with the aforementioned lens, at least in the optical axial direction. Here, the lens group means one or more pieces of lenses (for example, two pieces). Accordingly, in the present specification, numerical aperture NA of the objective lens on the optical information recording medium side (that is image side) means the numerical aperture NA of the lens surface of the objective lens positioned closest to the optical information recording medium side. Still further, in the present specification, necessary numerical aperture NA means the numerical aperture stipulated in the specification of each optical information recording medium, or it means the numerical aperture of the objective lens having a diffraction limit power that can obtain a spot diameter necessary for recording or reproducing information in accordance with the wavelength of the light source in use, for each optical information recording medium.
In the present specification, the second optical information recording medium means, for example, the optical disks of various kinds of CD systems such as CD-R, CD-RW, CD-Video, and CD-ROM, while the first optical information recording medium means the optical disks of various kinds of DVD systems such as DVD-ROM, DVD-RAM, DVD-R, DVD-RW, and DVD-Video. Further, the thickness t of the transparent base board in the present specification includes the case of t=0.